1. Field of the Invention
The present invention relates to a knock sensor and particularly to a nonresonant knock sensor mounted on, for example, a vibration generating part of an internal combustion engine or the like and for converting a knocking vibration of the internal combustion engine into an electric signal to detect it.
2. Description of the Related Art
A structure of a conventional knock sensor will be described with reference to the drawings. FIG. 8 is a sectional view showing an inner structure of a generally known knock sensor 70. A metal base 21 mounted on a vibration generating part of an internal combustion engine or the like by a bolt or the like is constructed of a cylindrical part 21b in which a bolt through hole 22 is formed and a disk-like flange part 21a formed at its end. A lower insulating sheet 7 is slid onto the cylindrical part 21b to come in contact with the flange part 21a, and the following are successively slid thereon.
That is, a lower terminal plate 5, a piezoelectric element 4 having whole surface electrodes 4a at its upper and lower surfaces, an upper terminal plate 6, an upper side insulating sheet 8, and a weight 9 are slid thereon. Next, a nut 23 is screwed onto a male screw part 21c threaded on the tip end outer peripheral surface of the cylindrical part 21b, and a tool such as a torque wrench is used to tighten the component parts to the flange part 21a at a predetermined torque. Thereafter, a terminal part 14 is bonded to the lower terminal plate 5 and the upper terminal plate 6 by soldering or resistance welding. The base 21 except the inner peripheral surface and both end surfaces of the cylindrical part 21b is coated with a resin mold to form a case 13, and a connector part 15 for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case 13.
This kind of knock sensor is attached to, for example, an internal combustion engine by a bolt inserted in the through hole 22 provided in the axial direction of the base 21. When a knocking vibration is generated in the internal combustion engine, the component members, such as the piezoelectric element 4 and the weight 9, constituting the knock sensor are vibrated integrally with the knocking vibration, this vibration is converted into a voltage signal by the piezoelectric element 4, and the detection signal is outputted from the terminal part 14 to the outside through the lower terminal plate 5 and the upper terminal plate 6 (see, for example, patent document 1).
[patent document 1] JP-A-2002-257624
The conventional knock sensor is constructed as described above, and since the electrodes 4a of the piezoelectric element 4 are provided on the whole surfaces in contact with the terminal plates 5 and 6, the electrostatic capacity of the piezoelectric element 4 is determined by the thickness of the piezoelectric element and a level corresponding to the area of the whole surface electrode, and the level of the output signal extracted from the knocking vibration is also determined to a predetermined level. In order to change the level of the output signal extracted from the knocking vibration, it is necessary to change the thickness of the piezoelectric element 4 or its diameter. In this case, there also arises a necessity to change the outer shape of the knock sensor.
Thus, there arises a necessity to partially provide electrodes without providing the whole electrodes on the surfaces of the piezoelectric element 4 coming in contact with the terminal plates 5 and 6. However, it has been found that when the piezoelectric element 4 is made to have partial electrodes, since a gap corresponding to the thickness of the partial electrode exists between the piezoelectric element 4 and the terminal plates 5 and 6, there are following problems.
That is, in the piezoelectric element 4, when a polarization processing of the partial electrode part is performed, the polarization action is also exerted on an electrodeless part around the partial electrode. Although electric charges generated by a pyro (pyroelectric) effect caused by environmental temperature change are successively discharged through the terminal plates 5 and 6 at portions where the electrodes exist, they are stored at portions where the electrodes do not exist. The electric charges stored in the electrodeless parts of the piezoelectric element produce an electrical breakdown by a voltage corresponding to the gap existing between the piezoelectric element 4 and the terminal plate 5 or 6, and are instantaneously discharged to the terminal plate 5 or 6.
The electric charges transferred to the terminal plate 5 or 6 by this discharge are applied to the piezoelectric element 4 in the form of a return current. At this time, the electric charge of the same polarity as the electrode polarity of the piezoelectric element 4 is applied, so that the piezoelectric element 4 is instantaneously expanded in the polarization direction, and an electric charge with reverse polarity is generated in the inside. As stated above, in the case where the electrode of the piezoelectric element is made the partial electrode in the structure of the conventional knock sensor, there is a problem that noise is superimposed on the output of the knock sensor.